Raised Bed Soil Calculator
A raised bed is filled by its inside dimensions, and the soil settles after watering — so this calculator gives you volume, bags, and a 60/30/10 topsoil, compost, and perlite mix split. Runs 100% in your browser.
Raised Bed Soil Calculator
About this raised bed soil calculator
Bed soil is length times width times depth, with a settling allowance and a mix split: cubic feet = length × width × depth in feet, cubic yards = cubic feet ÷ 27, bags = ceiling(cubic feet with allowance ÷ 1.5). A 4 × 8 ft bed at 12 inches deep is 32 cu ft net, 1.19 cu yd, and with 10% settling allowance that is 35.2 cu ft — 24 bags of 1.5 cu ft, split 60/30/10 into about 21.1 cu ft topsoil, 10.6 cu ft compost, and 3.5 cu ft perlite. Our raised bed soil guide covers depth by crop and the mix in detail.
Measure the inside, not the outside. A bed built from 2-inch boards loses 4 inches off each outside dimension, so an 8 ft outside bed is really about 7 ft 8 in of soil — and on a 4 ft wide bed that error alone is about 8% of the order. Depth is the other trap: 12 inches is the practical minimum for most vegetables (roots need room, and shallow beds dry out), while deep-rooted crops like carrots and parsnips want 18–24 inches, and the calculator's depth field is in inches for exactly that reason.
The 60/30/10 mix is the standard raised-bed recipe: topsoil for bulk and structure, compost for nutrients and water holding, perlite (or coarse vermiculite) for drainage and air. Pure topsoil compacts into a brick in a bed; pure compost shrinks dramatically as it breaks down and costs far more. If you are filling on top of existing soil, you can skip the bottom few inches of mix and let roots grow down into it — but do not count that as bed volume unless the ground below is already loose and fertile.
How to use
- Measure the inside length and width of the bed in feet (subtract board thickness from outside dimensions).
- Enter the soil depth in inches — 12 inches minimum for most vegetables, 18–24 inches for deep-rooted crops.
- Pick a settling allowance — 10% standard, 20% for loose fill or deep beds.
- Press Calculate for volume, bags, and the 60/30/10 mix split.
Good to know
- Rectangular beds only. Round beds, keyhole beds, and tiered beds need the area of each section measured separately and added.
- Bag size is fixed at 1.5 cu ft (the common bagged-soil size). If your supplier sells 2 cu ft or 1 cu ft bags, divide the with-allowance cubic feet by that size and round up.
- The 60/30/10 mix is a general vegetable-bed recipe. Native-soil beds, cactus/succulent beds, and acid-loving plants (blueberries, azaleas) need different mixes.
Formula & standards
Formula: Cubic feet = inside length × inside width × (depth ÷ 12); cubic yards = cubic feet ÷ 27; bags = ceiling(cubic feet with allowance ÷ 1.5).
Standard: None — the 60/30/10 topsoil/compost/perlite split is standard gardening practice, not a code.
Waste allowance: Fresh fill settles 10–20% after the first waterings; fill proud of the rim and top up after a week.
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
A 4 × 8 ft bed at 12 inches deep needs 32 cu ft net (1.19 cu yd). With 10% settling allowance that is 35.2 cu ft, or 24 bags of 1.5 cu ft soil.
What is the best soil mix for a raised bed?
The standard mix is 60% topsoil, 30% compost, and 10% perlite by volume: topsoil for structure, compost for nutrients, perlite for drainage.
How deep should soil be in a raised bed?
12 inches minimum for most vegetables; 18–24 inches for deep-rooted crops like carrots and parsnips. Shallow beds dry out faster.
Should I measure the inside or outside of a raised bed?
The inside — boards 2 inches thick steal 4 inches off each dimension, so an outside measurement overstates the soil by roughly 8% on a 4 ft wide bed.
Why does raised bed soil settle?
Fresh fill has air gaps that collapse after the first waterings, typically 10–20%. Fill proud of the rim and top up after a week.